General Background
There are many optical based approaches to identifying various parameters related to an object, such as an electronic component. The various parameters which may be reported include angular orientation of the component, location of features on the component (e.g., pick-up off-set), indication of the quality of the component, such as a bent lead (lead tweeze) in the case of a Quad Flat Pack (QFP) component, or a measurement of a feature located on the bottom of the component, such as height of a ball on a Ball Grid Array (BGA) component. The system for identifying these parameters is generally mounted in a pick and place machine which picks up the component and places the component onto a printed circuit board after determining proper orientation and pick-up offset of the component. All of these approaches rely on a quill in the pick and place machine to pick up the component and all generally utilize the motion control system in the pick and place machine in the measurement process. Some of the approaches use cameras to collect data representative of an image of the component and others collect data representative of an outline of the component. All have some sort of processor that analyzes the collected data in order to identify some specific portion of the component, such as the edges of the part or tips of the component leads.
The word "orientation" used with respect to an object, and forms thereof, will be used throughout this document to indicate the angular position of the object with respect to a reference. The word "location" used with respect to an object, and forms thereof, will be used throughout this document to indicate the (x,y) position of the object with respect to a nominal (x,y) reference location.
Different types of placement methodologies are now in use in pick and place machines as well. In general, pick and place machines can be characterized as being either "on-head" or "off-head". On-head sensor systems (considered together with their host pick and place machines) sense and correct for the orientation and pick-up offset of the component while the component travels to the target circuit board. On-head systems are preferred as they minimize the amount of time to place a component on a board, thereby increasing the throughput of the pick and place machine. Off-head sensor systems perform the data collection in a fixed location on the pick and place machine. With an off-head system, the quill must first move the component to a dedicated station and after acquiring the image, move the component to the target circuit board for placement. Off-head systems typically reduce throughput of the machine since the head makes additional stops or travels further, but are used because they are inexpensive, reduce the mass which must be carried on the head and one sensor can readily service multiple quills.
The essential purpose of a pick and place machine is to place an electronic component on a printed circuit board. Regardless of the type of sensor used to precisely locate certain features on the component, the host processor in the pick and place machine has an intended placement spot for the component. The location information from the sensor, vision camera or otherwise, is typically sent via a RS-422 port back to the host processor. Based on the sensor output and the desired placement position of the component, the host processor computes an orientation correction (x,y,.theta.) which includes an angular correction (.theta.) and a pick-up offset (x,y). Based on the orientation correction signal, the host processor instructs the motion control system in the pick and place machine to implement this correction movement, the motion control system responds, and then the host processor instructs the pick and place machine to place the component.
The electronic block diagram and general operational principles of a prior art shadow cast based sensor 2 and a prior art pick and place machine 12 is shown in FIGS. 1-2. A sensor system 8 consists of three parts; the sensor 2 (located on a sensor head, not shown), a controller module 4 and a two-wire coaxial cable 6 which connects sensor 2 to controller module 4. Sensor 2 is located on the sensor head of pick and place machine 12, while controller module 4 is located in a separate electronic enclosure, or alternatively, within machine 12. Operation of the system 8 is controlled by a processor 10 in pick and place machine 12, which generally controls the operation of machine 12 and issues commands to controller module 4 over communications bus 14, typically in a serial communications protocol of RS-232 or RS-422. A motion control system 16 controls x,y,z,.theta. movement of the sensor head via motors 16, 18, 20 and 22, respectively. Each motor has an associated encoder 24, 26, 28 and 30, respectively, each encoder adapted to independently output a signal representative of the change in x,y,z and .theta. position of the sensor head in pick and place machine 12. Pick and place machine 12 also includes an operator interface 32 which typically includes a CRT for viewing images of the component provided by the present invention, or optionally viewing summary results of operations of machine 12. A power supply 34 is located in pick and place machine 12, and supplies all the operating power required by sensor 2 over the bus 14, through controller module 4 and over two wire coaxial cable 6 to sensor 2.
The general operation of shadow cast sensor 2 is best explained in FIG. 2. A component 40 is secured via a vacuum nozzle 42 or the like to sensor head (not shown). A plurality of light rays 44 are cast onto component 40 from one of its sides and a linear image detector 46 on the other side of component 40 captures data representative of the shadow of component 40. Sensor 2 is shown with a laser diode source 48 for light 44, but any sort of high intensity light source may be used, regardless of whether the light is coherent or not. At several pre-selected angular positions of rotary motor 22, the sensor 2 collects data from the detector representative of the shadow of component 40. As appropriate for the type of component, the pre-selected angular positions do not have to be equally spaced over 360 degrees nor is component 40 required to spin at a constant angular velocity. For each preselected angular position, electronics in controller module 4 compute the width of the component's shadow on the detector and the location of shadow's center. For a rectangular component angularly rotated 180 degrees, a plot of shadow width v. angle will produce two minima; one corresponding to the width of the component and one corresponding to its length. Once the two minimum widths and their corresponding angles and shadow centers are known, the location of a feature on component 40 (with respect to an internal (x,y) reference in the pick and place machine) may be computed. The pick-up offset is calculated once the location of a feature and the internal (x,y) reference for a pick and place machine are known. This system does not provide any further information not already derived from the shadow edges, so that it is unable to identify misplaced or missing balls on BGAs or flip-chips, and cannot identify lead position errors for leaded components such as QFPs or TSOPs. This shadow casting sensor is shown in U.S. Pat. No. 5,278,634 to Skunes et al., assigned to CyberOptics.RTM. Corporation. Other applications of a shadow cast system include allowing the light to fall on the component while it is tilted, so as to be able to discern information about the leads on a component rather than exclusively the profile. The shadow cast approach takes advantage of the motion control system already installed on pick and place machine 12, which has an extremely accurate rotary motor 22 and encoder 30 for reporting the angular position of nozzle 42.
Since there is an electronics circuit board and a sensor associated with each of the effective data channels for placing components, it is cumbersome to change the electronics board whenever a difference type sensor is installed. Furthermore, the electronics board associated with each sensor in pick and place machine requires valuable board space and additional communications overhead is associated with each additional sensor/electronics board combination. Additionally, the amount of space consumed by the combination of the cabling between the board, the sensor and the host processor (which is not typically connected to the same card cage as the sensor electronics board) is excessive and a more space efficient solution is desired. In sum, a new interchangeable sensor which would allow various sorts of sensors to be installed in a pick and place machine without excessive downtime, which also provides for more efficient use of computations and communications capability within the pick and place machine and finally, takes up a reduced amount of space, is needed.